Helper-dependent phage P4 is a satellite virus depending on temperate phage P2 as a helper. P2 provides the proteins for satellite phage head and tail synthesis, as well as cell lysis. P4 causes P2 head proteins to form a small head which is not found during normal P2 head morphogenesis. Thus P4 causes specific head size determination. We have found a phage DNA binding protein which may cause head size determination, or which may be responsible for DNA maturation or the initiation of DNA replication. This protein will be purified and characterized. P4 induces an RNA polymerase activity which may initiate DNA replication by synthesizing polyriboguanylic acid. We will isolate nascent P4 DNA chains and determine whether they carry polyriboguanylic acid at their 5' ends. P4 can cause P2 genes to be expressed under conditions where they would normally remain repressed. This effect, called transactivation, involves the interaction of phage proteins with E. coli RNA polymerase. We will identify and characterize these P4 protein(s) which regulate P2 gene expression.